Before Serena Williams picked up a racket at London’s Andy Murray Arena last week, two questions hung over her return to tennis.
First: what would he do? He answered that, in his first competition in almost four years, by winning. The 44-year-old girl and her partner, 19-year-old Victoria Mboko, ended up coming third in their opening match of the Queen’s Club competition. Their victory was sealed by a 116-mile-per-hour serve from Williams that her opponents could not return.
The second will take longer to answer: Did the GLP-1 weight loss drug improve his performance?
Williams is a paid spokesperson for the phone company Ro, which deals in similar drugs. In the ad, he says that he lost 34 pounds with the help of GLP-1s. It’s unclear whether Williams is still taking her drug of choice, tirzepatide (her publicist declined to comment), which has helped her lose stubborn baby weight and control her cholesterol. Although there is no indication that he took it to gain a competitive advantage, Williams has also said that the drug improved his training and his game. The drug “helped me improve everything I was already doing – eating healthy and exercising, whether it was like a professional tennis player at the top level or going to the gym every day,” he said. People magazine in August.
In his comeback, Williams is the first professional athlete to publicize his use of GLP-1s. But the sports world is full of whispers that he is not the only one. The more athletes follow Williams’ lead, the more pressing the question of whether GLP-1s should be treated as performance-enhancing drugs.
The World Anti-Doping Agency, which sets the rules adopted by many international sports leagues, has said it is closely monitoring whether GLP-1 is abused; currently, athletes are free to take them. (A Ro spokesperson told me that “any patient receiving a GLP-1 prescription through Ro has been determined to be medically eligible for the treatment by a licensed medical provider.”) But experts are divided on whether GLP-1 has the potential to improve athletic performance. “I don’t see that there would be much benefit at all to using these substances for an athlete,” Thomas Hudzik, a pharmaceutical consultant who has served on an advisory group that recommends which drugs WADA should ban, told me. Meanwhile, Lars Engebretsen, head of WADA’s health, medical and research committee, told the Norwegian Broadcasting Corporation that he believes the drugs should be banned, but mostly because they can exacerbate eating disorders.
Opinions differ because no studies have yet tested these drugs in elite athletes. Williams’ return to tennis—which will continue with a doubles appearance with her sister Venus at Wimbledon—is one of the first opportunities, as far as we know, to see how an elite athlete performs after taking GLP-1. So far, results have been mixed: He won with Mboko at the Queen’s Club, but lost his first doubles match at the Berlin Open this week. Doubles also demands fewer athletes than singles, so Williams’ full potential has yet to be tested in competition. (Tennis fans are curious if Williams will get a wild-card chance at Wimbledon, too.) “The jury is out on endurance, stamina,” Rick Macci, who coached Williams during her childhood, told me.
Being trim can be an advantage in some sports. A light frame can reduce the amount of effort required for, say, climbing, running or gymnastics. And weight is built into the structure of other sports. Wrestlers, for example, must weigh in to qualify for their matches. A popular strategy is to get below the maximum weight for a given class—a process that GLP-1s can facilitate. “It makes perfect sense in weight class sports,” John Hawley, director of the Australian Catholic University’s Center for Metabolism and Performance, told me via email.
But whether GLP-1s will produce meaningful weight loss in athletes who are already in top shape is an open question. No large trials have tested GLP-1s in healthy people with a normal or low BMI, so it is unclear how much weight such people can lose on the drug. That could mean that among elite athletes, the drugs may have a significant effect only on a small group, including Williams, who are looking to get back into shape. Several clinical trials have also shown that GLP-1s can reduce markers of inflammation in the blood—but it remains unclear how much of that improvement is the result of weight loss or an independent effect of the drug.
Besides, being light doesn’t have to be high if it means less power. About 25 to 35 percent of the weight that GLP-1 patients lose is lean, according to a meta-analysis published in March. Athletes using GLP-1s will need to closely monitor their diet and training to ensure they maintain muscle (perhaps when dealing with the vomiting, nausea, and diarrhea often associated with GLP-1s). In addition, GLP-1 is believed to be a moisturizer dopamine signalwhich “may put a lid on effort and drive and competition,” Ziyad Al-Aly, a researcher at Washington University in St. Louis who has studied GLP-1s, told me. Lack of research is a problem here as well, Al-Aly emphasized: The best evidence for this hypothetical effect is a previous study that found that rats sought out their exercise wheel after being injected with semaglutide.
Some studies on the effects of GLP-1 in certain sports are currently underway. WADA, for example, is sponsoring an experiment to test how semaglutide changes the structure and performance of runners. But pharmaceutical companies are actively testing new types of weight loss drugs that may have different effects on muscles, as well as combinations of GLP-1 and other drugs intended to preserve energy. Some of these drugs, such as the class that exceeds the body’s natural limit on muscle growth, are already banned by WADA. Each new weight loss drug will confound any short-term understanding of how these drugs affect athletic performance.
Deciding whether a drug should be banned is not just a scientific endeavor: WADA bases its decisions in part on whether the use of drugs violates the “state of the game,” which it defines as “pursuing the values of human excellence through the self-sacrificing perfection of each Athlete’s natural talents.” Athletes are admired by us because they work so hard to acquire the best skills that most of us could never imagine having. We marvel at baseball sluggers because hitting a small ball that travels 90 miles an hour takes hard work; consistently hitting 400 feet is much more difficult. When a hitter takes an anabolic steroid, the wonder is largely lost.
Williams became a tennis icon in part because of her desire to compete. Even as a kid, “he was running so hard trying to get to the ball, he fell,” Macci said. When she competed at the Australian Open while pregnant in 2017, she defeated every opponent she faced, without dropping a set. Being 44 years old, having two children, and struggling with extra weight did not stop him from returning to professional tennis. Yes, he was probably lighter and faster because he took GLP-1. But a good return embodies the spirit of sports, too.
